The wide use of aluminum (Al) and its alloys to form structural members is due to advantages such as high strength to weight ratios, thermal and electrical conductivity, heat and light reflectivity and generally high corrosion resistance. However, aluminum is a very reactive metal, which is readily oxidized because of its high oxidation potential. The corrosion resistance of aluminum and its alloys in many environments is primarily due to the protective oxide film which rapidly attains a thickness of about 20 .ANG. on fresh metal exposed to either air or water. However, in the absence of an oxide film, the corrosion rate can be very high. Aluminum rapidly corrodes in environments of both high and low pH, which cause uniform dissolution of the oxide and opens the underlying metal to attack.
In natural environments such as air, fresh water, sea water and soils, the surface oxide film is so stable and adherent that aluminum is inherently corrosion resistant. However, other methods of corrosion protection are required in environments contaminated with chlorine, chloride or other highly corrosive and abrasive agents. Dry chlorine reacts with aluminum to form a stable chloride, AlCl.sub.3, which melts at about 192.degree. C. AlCl.sub.3 volatizes at relatively low temperatures because of its high vapor pressure. Consequently at 150.degree. C. aluminum can be consumed by chlorine attack at rate of 1 .mu.m or more per minute. Moreover, aluminum reacts more rapidly with moist chlorine than with dry chlorine. Therefore, aluminum and its alloys often fail in corrosive industrial environments in which high humidity and chlorine gas concentrations are employed.
There are a large number of surface treatments for corrosion protection of aluminum and its alloys. Commonly-employed methods include anodization and the use of organic coatings. Each of these procedures has its limitations. In anodization, aluminum is electrolytically oxidized and treated with hot water to form a nonporous coating of hydrated aluminum oxide thereon. Anodized aluminum is hard and resists corrosion, but not all aluminum alloys can be anodized to an acceptable appearance. Furthermore, anodizing is an energy intensive, difficult electrochemical process. Also, the brittle nature of the thicker films makes them susceptible to corrosion fatigue which causes local stress cracking and eventual rupture of the films. Clear organic coating compositions are an economically attractive choice for corrosion protection since they are easily handled and applied. But the organics are not effective for long term protection since gaseous molecules readily penetrate organic films and attack the underlying film-oxide interface.
Although tantalum pentaoxide films formed on polished Al single crystal surfaces by sputter deposition of tantalum and subsequent anodization effectively protect the Al from corrosion by water vapor saturated with chlorine, aluminum alloy surfaces are not protected, since the anodic film formed over grain boundaries, processing lines, and emergent precipitates is only weakly adherent, thus providing loci for stress corrosion cracks.
Therefore, a need exists for a method to protect corrosion-prone surfaces, particularly aluminum surfaces, by the application of a thin, ductile, chemically inert, dense and structurally homogeneous film.